1. Field of the Invention
The present invention relates to the field of vacuum sputter coating apparatus and particularly relates to an improved sputtering process and apparatus, and more particularly, to the construction of an improved cathode/anode assembly which provides faster deposition rates, better uniformity in the material deposited and longer lasting targets which in turn permit longer run times between periods of downtime.
2. Description of the Prior Art
The present invention is directed to an improved magnetron sputtering apparatus. A typical magnetron sputtering device includes a vacuum chamber having an electrode contained therein, wherein the electrode includes a cathode portion, an anode portion and a target. A vacuum is drawn in the vacuum chamber followed by the introduction of a process gas into the chamber. Electrical power supplied to the electrode produces an electronic discharge which ionizes the process gas and produces charged gaseous ions from the atoms of the process gas. The ions are accelerated and retained within a magnetic field formed over the target, and are propelled toward the surface of the target which is composed of the material sought to be deposited on a substrate. Upon striking the target, the ions dislodge target atoms from the target which are then deposited upon the substrate. By varying the composition of the target, a wide variety of substances can be deposited on various substrates. The result is the formation of an ultra-pure thin film deposition of target material on the substrate.
What is termed the electrode above (which includes both a cathode portion and anode portion) is sometimes simply called a cathode by those skilled in the art. While the inventor recognizes that convention, the discussion below does not follow this convention because calling the electrode a cathode will only cause confusion in the following discussion which discusses separately the cathode and anode portions of the electrode.
Several problems exist with respect to known magnetron sputtering devices.
First, the sputtering process produces intense heat. In order to prolong the life of both the target and the sputtering device, the sputtering cathode, particularly the area behind the target, is typically cooled with water. In the known prior art sputtering devices, water simply enters a water chamber associated with the cathode and circulates around the circumference of the water chamber, and exits the water chamber. This method and apparatus for cooling the cathode is not particularly effective, and provides a significant limitation on the prior art magnetron sputtering devices because the power supplied, and in turn, deposition rates, must be held at a reduced level to avoid overheating the cathode assembly. Alternatively, if the power supplied exceeds this threshold, at substantially high operating temperatures due to inefficient cooling, cracks will form in highly stressed target materials, such as ceramics and brittle metals. Also, the heat buildup causes higher electrical resistance which impedes the flow of electrons which yields lower deposition rates than would otherwise have been possible if such heat were not present.
Typically in the prior art, an electrical power cable was attached to the water chamber base plate, and an electrical circuit to a cathode body traveled from the base plate through the water chamber sidewalls to the cathode body. The interface of the base plate and the sidewall, while sealed with an O-ring seal, still resulted in the formation of corrosion and poor electrical contact degrading the electrical circuit due in part to an oxidation buildup at the base plate/sidewall interface. This resulted also in unsteady process parameters and difficulty in obtaining RF matching.
Targets of the prior art are typically held within or adjacent to the electrode by one or more target clamps. In the prior art, such clamping mechanisms were quite bulky, resulting in a much larger diameter electrode than would have otherwise been necessary to accommodate the bulky clamping mechanism. Further, prior art clamping mechanisms are not able to adjust to a wide range of target thicknesses and many involved the use of small screws to hold the clamping mechanism in place, which screws were difficult to start, and easy to strip and/or lose.
Further, in electrodes of the prior art, an anode shield was employed which was also bulky in design, also resulting in a much larger overall size than the given target area. Typically, the prior art anode shield was fixed to a water chamber base plate, and a clamping mechanism was attached at the opposite end with screws or welds to provide an anode shield. Further, the anode-to-cathode spacing was fixed and not adjustable. A smaller overall size is desirable because for a given target size it permits closer proximity of the cathode to the substrate, which yields more uniform depositions on the substrate. The lack of welds is desirable, as welds can cause fluctuations in the magnetic fields which is undesirable. Eliminating the screws permits smaller overall dimensioning and eliminates the starting, stripping and/or misplacement of screws.
Prior art electrodes were maintained in a gas chamber, in which a process gas was injected which could then be ionized. Higher gas pressures and more volume of gas were required than were truly necessary because the ionizing gas was really necessary only over the target area, and not within the entire chamber. An additional limitation with the prior art electrodes included the fact that the use of higher volumes of gas also resulted in a higher ratio of gas inclusions and defects on the substrate during the coating operation resulting in films with less than desired uniformity.
Prior art magnets were comprised of a series of separate individual magnetic pieces. These individual magnetic pieces resulted in magnetic field fluctuations between the pieces, which resulted in less than uniform magnetic fields and an inefficient use of the target area because the magnetic field was not sufficiently uniform to use all or nearly all of the target area.
In addition, the use of individual magnetic pieces further required the use of an additional device known as a magnetic "shaping ring" that increased the size and cost of the electrode. This ring, placed between the magnets and the target area, functioned in the prior art to shape and unify the magnetic field. However, it also weakened the field in doing so and moved the magnets further away from the target.
Elimination of the shaping ring would permit a much closer target-to-magnet distance which would allow more magnetic field to "passivate" or flow through the target area and the stronger magnetic field would permit much higher deposition rates and greater plasma density. Plasma density here refers to the number of gaseous ions retained within the magnetic field. The higher the field strength, the fewer gaseous ions can escape the magnetic field, or conversely, the more ions are retained within the field. Increasing the number of ions within the magnetic field is referred to as increasing the plasma density. With increased plasma density, higher power can be supplied allowing higher deposition rates.
In addition to increasing the target-to-magnet distance, the magnetic shaping ring has the additional disadvantage of "soaking up" or dissipating the magnetic field which reduces the strength of the overall magnetic field. The weaker magnetic field permits the undesirable consequence of the escape of both gaseous ions and secondary electrons from the magnetic field. Secondary electrons are created during the ionizing process when the gaseous atom is ionized to form the gaseous ion and accompanying secondary electrons according to standard charge balancing theory.
Importantly, the presence of the magnetic shaping ring substantially limited the prior art's ability to sputter magnetic materials to only very thin magnetic materials, such as foils and the like, on the order of 3 to 7/1000ths in thickness. Certainly, targets having the standard target thickness of approximately 1/8 to 1/4 inch could not be composed of magnetic material in the prior art due to the presence of the magnetic shaping ring. Magnetic materials could not be sputtered for three reasons. First, the magnetic shaping ring forced a larger target-to-magnet distance as described above weakening the magnetic field over the target area. It should be noted that with each incremental increase in target-to-magnet distance, the magnetic field was reduced two times in strength and; therefore, minor differences in target-to-magnet distances resulted in significant field strength loss. Second, the magnetic shaping ring soaked up part of the magnetic field itself, as described above. Finally, the magnetic target material shunted whatever magnetic field strength was left over the target area. For most applications, at least 300 gauss field strength was needed over the target area with higher field strengths being desired as described above. With prior art electrodes utilizing non-magnetic target materials, this threshold could be reached. However, when magnetic target materials were substituted for the non-magnetic materials, this threshold could not be reached unless the magnetic target material was very thin, in the nature of a foil as described above.
In the prior art, simple rectangular magnets were used. These rectangular magnets resulted in magnetic flux lines which tended to "dig out" the target area in a rather steep valley-like formation exhibiting a steep erosion profile in the magnetic null point area. The target area would rapidly wear thin in the null point area, but much target material remained that was therefore not utilized outside of this area. This also resulted in much shorter process run times, as the target would wear more quickly and the process would have to stop while the target was being replaced.
A need exists in the art for a magnetron sputtering device and process for sputtering wherein the sputtering electrode is more efficiently and effectively cooled, and wherein electrical connections are not susceptible to degradation due to oxide formations. Further, a need exists in the art for a magnetron sputtering device and process for sputtering wherein the overall electrode size relative to target area is reduced and wherein both the anode and cathode shields are easily adjusted and wherein the target area is easily retained and replaced, and wherein the anode-to-cathode spacing may be adjusted. A need also exists in the art for a magnetron sputtering device and process for sputtering wherein a minimum of ionizing gas is used. Finally, the need exists in the art for a magnetron sputtering device and process for sputtering wherein the magnetic shaping ring can be eliminated and wherein targets of standard thicknesses, such as 1/8 to 1/4 inch in cross section composed of magnetic materials, can be sputtered.